The infamous IP aliasing code for ppp, modified to work as a runtime option

(otherwise ppp's behavior remains unchanged) and documented by myself,
Steve Sims, Nate Williams, Martin Renters and god-only-knows who else. :-)
Submitted by:	nate
Obtained from:	Charles Mott <cmott@srv.net>

1 files changed, 42 insertions, 3 deletions

diff --git a/usr.sbin/ppp/ppp.8.m4 b/usr.sbin/ppp/ppp.8.m4
index 8deb7e3..7a4227b 100644
--- a/usr.sbin/ppp/ppp.8.m4
+++ b/usr.sbin/ppp/ppp.8.m4
@@ -1,5 +1,5 @@
 .\" manual page [] for ppp 0.94 beta2 + alpha
-.\" $Id: ppp.8,v 1.19 1996/10/31 19:55:05 joerg Exp $
+.\" $Id: ppp.8,v 1.20 1996/12/03 21:38:52 nate Exp $
 .Dd 20 September 1995
 .Os FreeBSD
 .Dt PPP 8
@@ -9,7 +9,8 @@
 Point to Point Protocol (aka iijppp) 
 .Sh SYNOPSIS
 .Nm
-.Op Fl auto | ddial | dedicated | direct
+.Op Fl auto | ddial | direct | dedicated 
+.Op Fl alias
 .Op Ar system
 .Sh DESCRIPTION
 This is a user process
@@ -53,13 +54,21 @@ will act as a daemon and wait for a packet to be sent over the
 link.  When this happens, the daemon automatically dials and establishes the
 connection.
 
-In almost the same manner ddial mode (dedicated dialing or demon dialing)
+In almost the same manner ddial mode (dedicated or demon dialing)
 also automatically dials and establishes the connection.  However, it
 differs in that it will dial the remote site any time it detects the
 link is down, even if there are no packets to be sent.  This mode is
 useful for full-time connections who worry less about line charges
 and more about being connected full time.
 
+.It Supports packet aliasing.
+Packet aliasing, more commonly known as masquerading, allows computers
+on a private, unregistered network to access the internet.  The
+.Em PPP
+host acts as a masquerading gateway.  IP addresses as well as TCP and
+UDP port numbers are aliased for outgoing packets and de-aliased for
+returning packets.
+
 .It Supports server-side PPP connections.
 Can act as server which accepts incoming
 .Em PPP
@@ -380,6 +389,36 @@ You must use
 .Dq quit all
 to terminate the program as well.
 
+.Sh PACKET ALIASING
+
+The
+.Fl alias
+command line option enables packet aliasing.  This allows the
+ppp host to act as a masquerading gateway for other computers over
+a local area network.  Outgoing IP packets are are aliased so that
+they appear to come from the ppp host, and incoming packets are
+de-aliased so that they are routed to the correct machine on the
+local area network.
+
+Packet aliasing allows computers on private, unregistered
+subnets to have internet access, although they are invisible
+from the outside world.
+
+In general, correct ppp operation should first be verified
+with packet aliasing disabled.  Then, the 
+.Fl alias
+option should be switched on, and network applications (web browser,
+telnet, ftp, ping, traceroute) should be checked on the ppp host.
+Finally, the same or similar applications should be checked on other
+computers in the LAN.
+
+If network applications work correctly on the ppp host, but not on
+other machines in the LAN, then the masquerading software is working
+properly, but the host is either not forwarding or possibly receiving
+IP packets.  Check that IP forwarding is enabled in /etc/sysconfig
+and that other machines have designated the ppp host as the gateway
+for the LAN.
+
 .Sh PACKET FILTERING
 
 This implementation supports packet filtering. There are three kinds of